Commutating capacitor devices constitute pluralities of capacitors which are connectable, in sequence, in series with or in shunt across a signal path to simulate the impedance effect of filters in the band-rejection or bandpass mode. One such device, called a commutating capacitor triplet, and described in U.S. Pat. No. 3,729,695, issued to J. H. Condon on Apr. 24, 1973, contains three capacitors which, during each cycle of the input wave, are connected in pairs to create six sequential states. Each capacitor pair is series connected across the input wave and is charged by the input wave (and the wave is thus sampled) at a particular phase of each wave cycle at an instant of time which is separated by 60 degrees from the next prior and from the next subsequent sampling.
Prior art patents disclosing commutating capacitive devices include U.S. Pat. No. 3,403,345 issued Sept. 24, 1968 to R. L. Frank and A. H. Phillips, U.S. Pat. No. 3,758,884 issued Sept. 11, 1973 to L. G. Bahler and J. H. Condon and U.S. Pat. No. 3,795,877 issued Mar. 5, 1974 to M. A. Poole.
The Frank et al patent discloses two series connected commutating capacitive devices wherein one device is driven substantially in phase with and the other substantially in phase quadrature with certain input signal components. The output of these series devices is subtractively combined with an attenuated portion of the input signal to substantially eliminate the above-mentioned signal components.
The Bahler et al patent discloses a band-rejection filter which comprises a plurality of commutating capacitive devices connected in parallel and driven in different phases with respect to one another wherein the outputs are additively combined.
The Poole patent discloses a notch filter comprising a first commutating capacitive device series connected with a second commutating capacitive device wherein the series output is fed back to the output of the first device in the series.
A useful implementation utilizing a commutating capacitor device might be a peak or envelope detection arrangement. The capacitor device is placed across the input signal and thus arranged in the bandpass mode. One sampling state of the input wave occurs at or near the signal peak of the wave cycle and, assuming that the commutator control signal frequency is the same as the input signal frequency, subsequent samplings in the corresponding state occur at the same place of subsequent input wave cycles whereby a relatively fixed charge of the signal peak samplings is obtained for that one state. Peak detection of the various states then indicates the charge of this one state as being the signal peak.
In the event that the frequency (and phase of the input signal) should drift with respect to the control signal, corresponding samplings shift in phase and the amplitude of each of the charges is thus shifted. Successive ones of the peaks of the input wave may, or may not, be sampled, because different phases of the input signal are being sampled. Thus different peak signal amplitudes of the wave cycles are sampled and a peak detection of the various samples result in a ripple output. A reduction of envelope or peak detection ripple might be accomplished by increasing the samplings per wave cycle to reduce the phase angle between samples. This, however, normally requires an increase in the number of capacitors.
It is the object of this invention to reduce output ripple without increasing the number of capacitors.